The present invention concerns water-based self-cross-linkable resins for use in the application of coatings to substrates, a method of preparing such resins and coating compositions containing the resins. The present invention also concerns a method for making low temperature cured, cross-linked coatings from the self-cross-linkable material of the invention.
Coating systems based on organic solvent-based materials, such as isocyanate systems, are available which provide high performance, urethane cross-linked coatings but engender environmental and fire hazards because of the use of volatile or toxic organic solvents. Commercially available isocyanate compounds typically are toxic and highly reactive, requiring the taking of suitable precautions in handling and storing the same. Aqueous solutions or dispersions of polyurethanes for coatings are known, but these known systems usually require high curing temperatures on the order of 350.degree. to 600.degree. F. (176.degree. to 315.degree. C.) in order to obtain cross-linking through the urethane groups. Although other low temperature-, or even room temperature-curable aqueous solutions or dispersions of isocyanate-free polyurethanes are available, such coatings do not cross-link through urethane groups and therefore are not likely to meet the performance standards attainable by urethane cross-linked coatings. Most aqueous dispersions of polyurethanes are usually attained by the addition of acids to form cationic dispersions or by the addition of bases to form anionic dispersions, or by the addition of surfactants, all of which additives can adversely affect the properties of the cured film obtained thereby. For example, the aqueous cationic-, anionic-, or surfactant-dispersed isocyanate-based polyurethanes often suffer from a lack of stability upon aging. If there are -NCO groups present, a reaction between water and the isocyanate will usually take place within about three to twenty hours at room temperature. Thus, isocyanate-based polyelectrolytes which are fully solule in water either readily hydrolyze in water of, after removal of water, become brittle and hygroscopic. Because of these drawbacks occasioned by the high ion group content of such materials, they are not of significant practical importance in the field of coatings and plastics generally.
One class of non-ionic aqueous solutions of polyurethanes is based upon the incorporation of polyester-glycol or polyether-glycol segments. However, the polyester-glycol types are sensitive to hydrolytic degradation while the water solubility of the polyether-glycol based resins is isocyanate-dependent. Moreover, both types tendto yield cured films with excessive sensitivity to water, i.e., films which are subject to swelling, turbidity (turning white), softening, and variable adhesion upon exposure to water.
Numerous literature references exist showing the reactions of primary and secondary amines with, for example, propylene carbonate to yield corresponding hydroxypropyl carbamates (Compt. rend. 1142, 1954). The literature also shows that bishydroxyalkyl carbamates derived from corresponding diamines have been further self-condensed, or transesterified with other diols, to produce linear thermoplastic polyurethanes. For example, see the article "The Preparation of Polymeric and Cyclic Urethans and Ureas from Ethylene Carbonate and Amines" by Elizabeth Dyer and Harvey Scott, J.A.C.S. (1956) pp. 672-675. See also the report "Polyurethane elastomers obtained without the use of diisocyanates" by L. Ya. Rappoport, G. N. Petrov, I.I. Trostyanskaya and O. P. Gavrilova in International Polymer Science and Technology, 8, No. 1, 1981 and an article by Richard D. Cowell entitled: "Thermoplastic Polyurethane Elastomers: Chemistry Properties and Processing for the 80's" in the Journal of Elastomers and Plastics, Vol. 14, (October, 1982) pages 195-203.